When scrap metal is heated to a liquid, molten state, certain impurities may be separated from the molten metal by the introduction of conventional fluxes which react with the impurities to form what are conventionally known as furnace slags. These slags rise to the surface and float on top of the molten metal.
Slag is of little or no value in making use of the molten metal. To the contrary, slag can interfere with using alloy additives to make various metal specifications.
For example, in making alloy steel, soluble oxygen is an unwanted contaminant. Slag which rises to the top of molten steel contains a large amount of soluble oxygen. If slag is present when alloys are added to the molten steel, then the soluble oxygen in the slag will react with the alloys and inhibit the alloys from reacting with the molten steel. Thus, the slag inhibits the alloying process. Also, the presence of slag in the molten steel facilitates the formation of particulate inclusions which, if large enough, may be detrimental to the physical properties of the steel.
Since furnace slag is a contaminant which may have a deleterious effect on making alloy steels, it is desirable to remove the slag before alloys are added to the molten metal. Slag removal is usually done before alloys are added to the molten steel. Any slag which is removed is usually discarded. The process of removing slag from molten steel is often known as slag control.
Slag control has been a particularly difficult problem when scrap steel is melted in tilting furnaces. As discussed below, there have been numerous attempts at controlling slag which separates from molten steel that is discharged from a tilting furnace.
The typical tilting furnace is mounted on a tilting platform. A tap hole is located on the side of the furnace. A trough is mounted on the side of the furnace, just below the tap hole.
When the furnace is heated, scrap steel in the furnace melts into a molten liquid state. Slag will separate from the molten steel and float in a separate layer on top of the molten steel. The level of the floating slag is usually kept below the level of the tap hole when the furnace is upright.
When the furnace is tilted, the operator of the furnace will attempt to tilt the furnace sufficiently so that the molten steel flows through the tap hole and the slag floats at a level above the level of the tap hole. As the molten steel drains from the furnace, the operator increases the angle of tilt in order to keep the slag at a level above the level of the tap hole. Thus, the operator attempts to cause all of the molten steel to flow through the tap hole before the slag begins to flow through the tap hole.
While molten steel flows through the tap hole, some slag will flow with the molten steel through the tap hole when a vortex forms. The vortex draws a very fluid layer of floating slag, known as interface slag, through the tap hole while molten steel is flowing through the tap hole. This interface slag floats in a layer between the molten steel and the rest of the floating slag. It has much less viscosity, and a higher concentration of soluble oxygen, than the rest of the floating slag. It is particularly deleterious to the alloying process.
The operator cannot see the vortexing of this interface slag because the furnace is usually enclosed on all sides and the top. Therefore, there is very little that he can do to prevent this interface slag from contaminating the molten steel during the process of pouring the molten steel through the tap hole. The pouring or tapping process is conventionally known as a "tap."
Near the end of the tap, the level of the molten metal and floating slag in the furnace has fallen so that the floating slag is at the level of the tap hole inside the furnace. The floating slag will thus begin to flow through the tap hole and contaminate the molten steel which has already been poured from the furnace. At this point, the operator attempts to stop the tapping process quickly by closing the tap hole and/or returning the furnace to the upright position.
However, because a tilting furnace is usually fully enclosed, the operator usually cannot see inside the furnace to determine exactly when the slag is about to begin flowing through the tap hole. Therefore, the operator usually waits until he sees slag coming out of the tap hole and into the trough before attempting to stop the flow of slag and returning the furnace to the upright position.
Thus, the traditional method of slag control consists of closing the tap hole and/or returning the furnace to the upright position after slag is observed to begin flowing through the tap hole and in the trough. As discussed below, there have been numerous attempts to implement this basic method of slag control on tilting furnaces, including Vost-Alpine slag stoppers, the E-M-L-I system, and various tap hole gates.
The Vost-Alpine slag stopper is a large, articulating nitrogen gas cannon which is used to close the tap hole. Operating under very high pressure, the cannon discharges nitrogen gas into the tap hole of the furnace on demand, and this stops the flow of molten steel and slag through the tap hole. Thus, the Vost-Alpine slag stopper is a kind of tap hole gate.
The E-M-L-I system consists of an electronic sensor which is mounted to the furnace inside the tap hole refractory. The sensor can sense when a predetermined percentage of slag is contained in the molten metal which is flowing through the tap hole. When the predetermined percentage is sensed by the sensor, the sensor communicates this to the operator of the furnace, who will then return the furnace to the upright position. Thus, the E-M-L-I system is used to control slag by directing the operator of the furnace to stop flow through the tap hole as soon as a minimum amount of slag begins to flow through the tap hole.
A variety of mechanical tap hole gates are used to control slag. The gates have a variety of shapes including the shapes of a tetrahedron or globe (also known as "cannonball"). The gate may slide into position via a rotary mechanism.
The eccentric bottom tapping gate is another attempt at slag control in an electric arc furnace. It requires that the tap hole be made in the bottom, rather than the side, of the furnace. When the operator observes slag pouring from the furnace, he closes a sliding gate to block the tap hole and prevent further flow through the tap hole. This method of slag control is quite expensive in that it requires new furnace and ladle transfer cars or turrets to receive the molten steel tap discharge and the furnace no longer tilts. The ladles must be removed from the side of the furnace and placed underneath the bottom of the furnace.
None of these prior methods of slag control on a tilting furnace have performed particularly well. None of them solve the problem of contamination of the molten steel with slag which comes through the tap hole at the end of a tap before the operator can react to stop flow through the tap hole. None of them solve the problem of contamination of the molten steel with interface slag which vortexes through the tap hole while molten steel is flowing through the tap hole at the same time.
In the prior art known to the inventor, there is no known method or apparatus to control slag after it escapes through the tap hole of a tilting furnace. All of the prior art methods and apparati known to the inventor have simply attempted to stop flow through the tap hole when it is determined that all of the molten steel has come through the tap hole and floating slag is beginning to flow through the tap hole. None of these prior art methods and apparati control or remove the slag after it goes through the tap hole and into the trough.
It would be desirable to control slag in a tap discharge of molten metal after it flows through the tap hole and before it flows out of the trough and into the ladle, wherein the slag control apparatus tilts with a tilting electric arc furnace and positive separation and control of the slag, including interface slag, is established. Further, it would be desireable to view the level of molten metal and floating slag in the discharge trough in order to coordinate the separation, retention and discharge of the slag with the tilting of the furnace in a positive manner, with an apparatus which can be removed and replaced as necessary, without removal or replacement of the discharge trough or furnace.